Uros Mestric

Coherent and quasi-coherent variability in active galactic nuclei (AGN) can serve as a useful diagnostic of dynamical processes operating in the immediate environments of supermassive black holes. The time-domain capabilities of the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) enable systematic searches for such signals across large AGN samples. We identify and classify coherent oscillatory processes in AGN cores using LSST Data Preview 1 (DP1), and compile the results in the QhX Hub, an interactive catalog providing statistical variability features and visualization tools. AGN are selected by cross-matching LSST DP1 sources with external catalogs, and light curves are analyzed with the multiband wavelet-based Quasar harmonic eXplorer ( QhX ), designed to detect coherent signals in irregularly sampled, red-noise-dominated data. The catalog contains 230 objects stratified into variability classes. A single strong coherent candidate is identified: 3D-HST 2351, z = 0.198, LSST DP1 object_id = 611255072642321324, with a period of P = 2.7 6 − 0.06 + 2.65 days, where the large upper bound reflects the limited observational baseline. Its coherent nuclear variability is the primary motivation for follow-up. In subsequent LSST data releases, all catalog sources will be recalculated on progressively longer baselines, enabling more robust population-level characterization of coherent AGN variability.

We provide a comprehensive analysis of Ion3, the most distant LyC leaker at z = 3.999, using multiband HST photometry (F390W, F814W, and F140W) and reevaluated X-Shooter spectroscopy. Deep HST F390W imaging enables us to probe uncontaminated LyC flux blueward of ∼880 Å. In this work, the nonionizing UV 1500 Å/2800 Å flux was probed with the F814W/F140W band. High angular resolution allowed us to properly mask low-z interlopers and prevent contamination of measured LyC radiation. We confirm the detection of LyC flux at a signal-to-noise ratio of S/N ∼ 3.5 and estimate the escape fraction of ionizing photons to be in the range f_esc, rel = 0.06–1, depending on the adopted IGM attenuation. A morphological analysis of Ion3 reveals a clumpy structure made up of two main components, Ion3_A and Ion3_B, with effective radii of R_eff ∼ 180 pc and R_eff < 100 pc, respectively, along with a total estimated delensed area in the rest-frame 1600 Å of 4.2 kpc². We confirm the presence of faint ultraviolet spectral features, including HeIIλ1640, CIII]λ1907,1909, and [NeIII]λ3968, with a rest-frame equivalent width EW(HeII) =(1.6 ± 0.7) Å and EW(CIII]) =(6.5 ± 3) Å. From the [OII]λλ3726,3729 and [CIII]λ1909/CIII]λ1906 doublets, we derived electron densities of n_e^[OII] = 2300 ± 1900 cm⁻³ and n_e^[CIII] > 10⁴ cm⁻³, corresponding to an interstellar medium (ISM) pressure log(P/k) > 7.90. Furthermore, we derived an intrinsic SFR(Hα) ≈77 M_⊙ yr⁻¹ (corresponding to Σ_SFR = 20 M_⊙ yr⁻¹ kpc⁻² for the entire galaxy) and subsolar metallicity 12 + log(O/H) = 8.02 ± 0.20 using the EW(CIII]) as a diagnostic. The detection of [NeIII]λ3968 line and [OII]λλ3726,3729 offer an estimate of the ratio [OIII]λ5007/[OII]λλ3727,29 of O32 > 50 and a high ionization parameter, log U > −1.5, using empirical and theoretical correlations. These measurements imply remarkably high ionization levels and density conditions produced by an ongoing bursty star formation, observed during an ionizing, optically thin phase of the ISM along the line of sight.

We present a detailed JWST/NIRSpec and NIRCam analysis of a gravitationally lensed galaxy (μ = 17 − 21) at a redshift of 6.14 magnified by the Hubble Frontier Field galaxy cluster MACS J0416. The target galaxy is a typical compact and UV-faint (M_UV = −17.8) Lyman-α emitter, yet the large magnification allows the detailed characterization of structures on sub-galactic scales (down to a few parsecs). Prominent optical Hα, Hβ, and [OIII]λλ4959, 5007 lines are spatially resolved with the high-spectral-resolution grating (G395H, R 2700), with large equivalent widths, EW(Hβ+O III) ≳ 1000 Å, and elevated ionizing photon production efficiencies, log(ξ_ion/erg⁻¹Hz) = 25.2–25.7. NIRCam deep imaging reveals the presence of compact rest-UV-bright regions along with individual star clusters of R_eff = 3 − 8 pc in size and M ∼ 2 ⋅ 10⁵ − 5 ⋅ 10⁶ M_⊙ in mass. These clusters are characterized by steep UV slopes, β_UV ≲ −2.5, which in some cases are associated with a dearth of line emission, indicating possible leaking of the ionizing radiation, as also supported by a Lyman-α emission peaking at ∼100 km s⁻¹ from the systemic redshift. While the entire system is characterized by low metallicity, ∼0.1 Z_⊙, the NIRSpec-IFU map also reveals the presence of a low-luminosity, metal-poor region with Z ≲ 2% Z_⊙, which is barely detected in NIRCam imaging; this region is displaced by > 200 pc from one of the brightest structures of the system in UV, and would have been too faint to detect if not for the large magnification of the system.

We report the serendipitous discovery of a faint (M_UV > −12.2), low-metallicity (Z ∼ 0.02 Z⊙) ionizing source, dubbed T2c, with a spectroscopic redshift of z = 6.146. T2c is part of a larger structure amplified by the Hubble Frontier Field galaxy cluster MACSJ0416 and was observed with the James Webb Space Telescope (JWST) NIRSpec integral field unit. Stacking the short-wavelength NIRCam data reveals no stellar continuum detection down to a magnitude limit of m_UV ≃ 31.0 (3σ). However, prominent Hβ, [O III]λλ4959, 5007, and Hα emissions are detected, with equivalent widths exceeding 200 Å, 800 Å, and 1300 Å (3σ), respectively. The corresponding intrinsic (magnification-corrected ×23 ± 3) ultraviolet and optical rest-frame magnitudes exceed 34.4 and 33.9 (corresponding to M_UV and M_opt fainter than −12.2 and −12.8 at λ_rest ∼ 2000 Å and ∼5000 Å, respectively), suggesting a stellar mass lower than a few 10⁴ M_⊙ under an instantaneous burst scenario. The inferred ionizing photon production efficiency (ξ_ion) is high: ξ_ion ≳ 26.08(25.86) 3(5)σ, assuming no dust attenuation and no Lyman continuum leakage. This indicates the presence of massive stars despite the low mass of the object. The very poor sampling of the initial mass function in such a low-mass star-forming complex suggests that the formation of very massive stars might be favored in very low-metallicity environments. T2c is surrounded by Balmer and weak oxygen emission on a spatial scale of a few hundred parsecs, after correcting for lensing effects. This system resembles a H II region potentially powered by currently undetected, extremely efficient, low-metallicity star complexes or clusters. We propose that massive O-type stars populate these low-mass, low-metallicity, high-redshift satellites, likely observed in an early and short formation phase, and contribute to the ionization of the surrounding medium.

We report the serendipitous discovery of a faint (M_UV > -12.2), low-metallicity (Z ~ 0.02 Zsun), ionizing source (dubbed T2c) with a spectroscopic redshift of z=6.146. T2c is part of a larger structure amplified by the Hubble Frontier Field galaxy cluster MACSJ0416, and was observed with JWST/NIRSpec IFU. Stacking the short-wavelength NIRCam data reveals no stellar continuum detection down to a magnitude limit of m_UV ~ 31.0 (3 sigma). However, prominent Hb, [OIII]4959,5007, and Ha emissions are detected, with equivalent widths exceeding 200A, 800A, and 1300A (3 sigma), respectively. The corresponding intrinsic (magnification-corrected x23 +/- 3) ultraviolet and optical rest-frame magnitudes exceed 34.4 and 33.9 (corresponding to M_uv and M_opt fainter than -12.2 and -12.8, at lambda_rest ~ 2000A and ~5000A, respectively), suggesting a stellar mass lower than a few 10^4 Msun under an instantaneous burst scenario. The inferred ionizing photon production efficiency (xi_ion) is high, xi_ion >~ 26.08(25.86) 3(5)sigma, assuming no dust attenuation and no Lyman continuum leakage, indicating the presence of massive stars despite the low mass of the object. The very poor sampling of the initial mass function at such low mass star-forming complex suggests that the formation of very massive stars might be favored in very low metallicity environments. T2c is surrounded by Balmer and weak oxygen emission on a spatial scale of a few hundred parsecs after correcting for lensing effects. This system resembles an HII region potentially powered by currently undetected, extremely efficient, low-metallicity star complexes or clusters. We propose that massive O-type stars populate this low-mass and metallicity high-redshift satellites, likely caught in an early and short formation phase, contributing to the ionization of the surrounding medium.

We present JWST/Near Infrared Spectrograph (NIRSpec) integral field spectroscopy (IFS) of a lensed Population III candidate stellar complex (dubbed Lensed And Pristine 1, LAP1), with a lensing-corrected stellar mass of ≲10⁴ M_⊙ and an absolute luminosity of M_UV > −11.2 (m_UV > 35.6), confirmed at redshift 6.639 ± 0.004. The system is strongly amplified (μ ≳ 100) by straddling a critical line of the Hubble Frontier Field galaxy cluster MACS J0416. Although the stellar continuum is currently not detected in the Hubble and JWST/Near Infrared Camera (NIRCam) and Near Infrared Imager and Slitless Spectrograph (NIRISS) imaging, arclet-like shapes of Lyman and Balmer lines, Lyα, Hγ, Hβ and Hα are detected with NIRSpec IFS with signal-to-noise ratios (S/N) of approximately 5 − 13 and large equivalent widths (> 300 − 2000 Å), along with a remarkably weak [O III]λλ4959, 5007 at S/N ≃ 4. LAP1 shows a large ionizing photon production efficiency, log(ξ_ion[erg Hz⁻¹]) > 26. From the metallicity indexes R23 = ([O III] + [O II])/Hβ ≲ 0.74 and R3 = ([O III]/Hβ) = 0.55 ± 0.14, we derive an oxygen abundance of 12 + log(O/H)≲6.3. Intriguingly, the Hα emission is also measured in mirrored subcomponents where no [O III] is detected, providing even more stringent upper limits on the metallicity if in situ star formation is ongoing in this region (12 + log(O/H) < 6). The formal stellar mass limit of the subcomponents would correspond to ∼10³ M_⊙ or M_UV fainter than −10. Alternatively, this metal-free, pure line-emitting region could be the first case of a fluorescing H I gas region induced by transverse escaping ionizing radiation from a nearby star complex. The presence of large equivalent-width hydrogen lines and the deficiency of metal lines in such a small region make LAP1 the most metal-poor star-forming region currently known in the reionization era and a promising site that may host isolated, pristine stars.

We present JWST/NIRSpec integral field spectroscopy (IFS) of a lensed Population III candidate stellar complex (dubbed Lensed And Pristine 1, LAP1), with a lensing-corrected stellar mass ~ -11.2 (m_UV > 35.6), confirmed at redshift 6.639 +/- 0.004. The system is strongly amplified (μ>~ 100) by straddling a critical line of the Hubble Frontier Field galaxy cluster MACS J0416. Despite the stellar continuum is currently not detected in the Hubble and JWST/NIRCam and NIRISS imaging, arclet-like shapes of Lyman and Balmer lines, Lya, Hg, Hb and Ha are detected with NIRSpec IFS with signal-to-noise ratios SNR=5-13 and large equivalent widths (>300-2000A), along with a remarkably weak [OIII]4959-5007 at SNR ~ 4. LAP1 shows a large ionizing photon production efficiency, log(ξ_{ion}[erg~Hz^{-1}])>26. From the metallicity indexes R23 = ([OIII]4959-5007 + [OII]3727) / Hb ~< 0.74 and R3 = ([OIII]5007 / Hb) = 0.55 +/- 0.14, we derive an oxygen abundance 12+log(O/H) ~< 6.3. Intriguingly, the Ha emission is also measured in mirrored sub-components where no [OIII] is detected, providing even more stringent upper limits on the metallicity if in-situ star formation is ongoing in this region (12+log(O/H) < 6, or Z < 0.002 Zsun). The formal stellar mass limit of the sub-components would correspond to ~10^{3} Msun or M_UV fainter than -10. Alternatively, such a metal-free pure line emitting region could be the first case of a fluorescing HI gas region, induced by transverse escaping ionizing radiation from a nearby star-complex. The presence of large equivalent-width hydrogen lines and the deficiency of metal lines in such a small region, make LAP1 the most metal poor star-forming region currently known in the reionization era and a promising site that may host isolated, pristine stars.

We investigate the blue and optical rest-frame sizes (λ ≃ 2300–4000 Å) of three compact star-forming regions in a galaxy at z = 4 strongly lensed (×30, ×45, and ×100) by the Hubble Frontier Field galaxy cluster A2744 using GLASS-ERS James Webb Space Telescope (JWST)/NIRISS imaging at 1.15 μm, 1.50 μm, and 2.0 μm with a point-spread function ≲0.″1. In particular, the Balmer break is probed in detail for all multiply imaged sources of the system. With ages of a few tens of Myr, stellar masses in the range (0.7–4.0) ×10⁶ M _⊙ and optical/ultraviolet effective radii spanning the interval 3 < R _eff < 20 pc, such objects are currently the highest-redshift (spectroscopically confirmed) gravitationally bound young massive star clusters (YMCs), with stellar mass surface densities resembling those of local globular clusters. Optical (4000 Å, JWST-based) and ultraviolet (1600 Å, Hubble Space Telescope–based) sizes are fully compatible. The contribution to the ultraviolet underlying continuum emission (1600 Å) is ∼30%, which decreases by a factor of 2 in the optical for two of the YMCs (∼4000 Å rest-frame), reflecting the young ages (−1. This galaxy would be among the extreme analogs observed in the local universe having a high star formation rate surface density and a high occurrence of massive stellar clusters in formation.

We present the results of a first search for galaxy candidates at z ∼ 9–15 on deep seven-band NIRCam imaging acquired as part of the GLASS-James Webb Space Telescope (JWST) Early Release Science Program on a flanking field of the Frontier Fields cluster A2744. Candidates are selected via two different renditions of the Lyman-break technique, isolating objects at z ∼ 9–11, and z ∼ 9–15, respectively, supplemented by photometric redshifts obtained with two independent codes. We find five color-selected candidates at z > 9, plus one additional candidate with photometric redshift z _phot ≥ 9. In particular, we identify two bright candidates at M _UV ≃ −21 that are unambiguously placed at z ≃ 10.6 and z ≃ 12.2, respectively. The total number of galaxies discovered at z > 9 is in line with the predictions of a nonevolving luminosity function. The two bright ones at z > 10 are unexpected given the survey volume, although cosmic variance and small number statistics limits general conclusions. This first search demonstrates the unique power of JWST to discover galaxies at the high-redshift frontier. The candidates are ideal targets for spectroscopic follow-up in Cycle-2.

Star-forming galaxies are the sources likely to have reionized the universe. As we cannot observe them directly due to the opacity of the intergalactic medium at z ≳ 5, we study z ∼ 3–5 galaxies as proxies to place observational constraints on cosmic reionization. Using new deep Hubble Space Telescope rest-frame UV F336W and F435W imaging (30 orbits, ∼40 arcmin², ∼29–30 mag depth at 5σ), we attempt to identify a sample of Lyman continuum galaxies (LCGs). These are individual sources that emit ionizing flux below the Lyman break (100% and therefore not physical. We are unable to confirm the true nature of these sources with the best available imaging and direct blue Keck/LRIS spectroscopy. More spectra, especially from the new class of 30 m telescopes, will be required to build a statistical sample of LCGs to place firm observational constraints on cosmic reionization.